Hologram information recording is a system for recording and reproducing information as two-dimensional page data and it is capable of improving the recording density and transmission speed sharply compared with a bit data system such as DVD. For this reason, intensive researches and developmental works are being conducted on hologram information recording as one of the next-generation optical information recording systems.
In particular, an optical information recording medium using a volume phase hologram recording material (hereinafter the former may also be referred to simply as hologram recording medium and the latter as hologram recording material) shows a diffraction efficiency of 1 which is the theoretical maximum and allows overwriting of information (multiple recording) and, because of these properties, the medium is expected to be put to practical use as a high-density information recording medium. Photopolymers are frequently used for hologram recording materials in consideration of simplicity in production of recording media and diversity in selection of raw materials.
Irradiation of a hologram recording medium containing a radically photopolymerizable component and a radical photopolymerization initiator simultaneously with a reference light and an information light, both consisting of a coherent active energy ray, generates an interference pattern composed of alternate light and dark areas; then, a polymerization reaction is induced to occur in the light areas and the polymerizable component diffuses in the direction to relax or destroy the concentration gradient formed by the polymerization reaction or diffuses from the dark areas to the light areas of the interference pattern. On the other hand, the non-reactive component diffuses in the direction to compensate for the diffusion of the polymerizable component, that is, in the reverse direction from the light areas to the dark areas of the interference pattern. In this manner, each component is distributed by concentration in the hologram recording layer in correspondence to the light intensity of the interference pattern and this concentration distribution is recorded as modulation structure of the refractive index.
The recording capacity of a hologram recording medium is, in principle, proportional to the thickness of a hologram recording layer. Therefore, it is advantageous that a hologram recording layer is formed as thickly as possible within a range where the influence of absorption of light by the material or the volume shrinkage accompanying the polymerization reaction on the recording performance can be substantially tolerated. The thickness demanded in practice for a hologram recording layer ranges approximately from 200 μm to 2 mm, and this is extremely thick compared with the conventional optical information recording media.
The patent document 1 discloses a substantially solid photopolymerizable composition which forms an image of refractive index upon exposure to a chemically-acting radiation, the said exposure constituting the one and only treating step, and substantially comprises 25-75% of a solvent-soluble, thermoplastic polymer binder (a), 5-60% of a liquid ethylenic unsaturated monomer (b), and 0.1-10% of a photoinitiator (c) which activates the polymerization of the unsaturated monomer upon exposure to a chemically-acting radiation. A solvent is used here to dissolve each component uniformly and to reduce the viscosity of the composition to such a degree as to allow application to a substrate. A hologram recording layer is formed by application of the composition to a substrate and, as a step for drying is required to remove the solvent by evaporation, the thickness of the recording layer has been limited substantially to 100 μm or below.
Several disclosures have been made on hologram recording materials capable of forming a hologram recording layer in a relatively large thickness amounting to approximately 200 μm or more without need of a solvent, hologram recording media using the same, and methods for preparing these materials and media. For example, some of such disclosures relate to a three-dimensionally crosslinked polymer matrix formed in-situ in the step for forming a hologram recording layer (the patent documents 2 to 4, the non-patent document 1, and elsewhere).
The three-dimensionally crosslinked polymer matrix here is considered to play roles of providing a hologram recording material with such physical strength as to maintain the shape as a hologram recording layer, of suppressing excessive migration of polymerizable compounds, and of reducing the volume shrinkage accompanying the polymerization during hologram recording (the non-patent document 1).
The patent document 2 discloses an optical product consisting of a three-dimensionally crosslinked polymer matrix and one kind or plural kinds of optically active monomers wherein the matrix polymer is compatible with a polymer formed by the polymerization of the optically active monomers. Further, an optical product is disclosed wherein the three-dimensionally crosslinked polymer matrix is formed by a polymerization reaction which is independent of the polymerization reaction of the optically active monomers.
The hologram recording medium of the aforementioned constitution does not require a solvent in the formation of a hologram recording layer and has an advantage in that a layer with a thickness ranging from several hundred microns to several millimeters can be formed relatively easily.
A hologram recording medium is required to be highly transparent. Therefore, it is necessary that the three-dimensionally crosslinked polymer matrix is compatible with the polymerizable monomers and with the polymers formed from the said polymerizable monomers.
However, this condition for compatibility is satisfied only by a limited number of polymer matrix-monomer combinations. Moreover, even a matrix polymer-monomer combination satisfying the condition for compatibility faces a problem that the difference in refractive index between the matrix polymer and the monomer or between the matrix polymer and the polymer formed from the said monomer cannot be made large.
In hologram information recording, the data recorded as an interference pattern are fixed completely by a treatment for fixation such as post-exposure. In continuous recording of a large volume of data, the time passing from the start of recording to the end of fixation becomes long and the data recorded earlier may in the meantime deteriorate. Therefore, a hologram recording medium is required to prevent the data recorded at least in the prescribed time from continuous recording to fixation from deteriorating (this property is hereinafter referred to as record holding property).
However, the problem with the optical products disclosed in the aforementioned patent documents is an insufficient record holding property.
The patent document 3 discloses a volume phase hologram recording material wherein the said material comprises a polymer matrix that has a three-dimensionally crosslinked structure containing a plurality of reactive groups and is capable of recording an interference fringe generated by the interference of a coherent light by the difference in refractive index and the material contains no polymerizable monomer as a component in hologram recording.
The patent document 4 discloses a photosensitive composition for volume phase hologram recording which comprises a compound having one or more active methylene groups in the molecule or a compound having two or more active methylene groups in the molecule, a compound having two or more groups to which an active methylene group or a carboanion formed therefrom adds nucleophilically, a Michael reaction catalyst, a photopolymerizable compound, and a photopolymerization initiator.
The hologram recording materials disclosed in the aforementioned patent documents show some improvement in the record holding property, but cannot be said to have sufficient sensitivity.
As described above, several hologram recording materials containing a three-dimensionally crosslinked polymer matrix formed in the step for forming a hologram recording layer have been disclosed thus far, but none has been offered that has high sensitivity, high contrast, and a good record holding property.
Patent document 1: JPH02-3081 A
Patent document 2: JPH11-352303 A
Patent document 3: WO2005/078531 A
Patent document 4: JP2005-275389 A
Patent document 5: JP2004-123873 A
Non-patent document 1: Chemistry of Materials, Vol. 12, pp. 1431-1438 (2000)
Non-patent document 2: Macromolecules, 34, pp. 396-401 (2001)